The present invention relates to operational amplifiers, and more particularly to an improved method of adjusting operational amplifier gain to reduce undesirable changes in bandwidth, output offset voltage and output noise due to gain change.
Gain adjustment of an operational amplifier can be accomplished by connecting a R-2R resistive ladder digital-to-analog converter, i.e., multiplying DAC, as an adjustable feedback element. When placed in the feedback loop the multiplying DAC permits adjustment of the feedback current, and therefor gain, for the operational amplifier. Conventionally the multiplying DAC is placed in the negative feedback loop.
Operational amplifier input offset voltages, offset voltage drift and noise are amplified by an amount determined by the closed loop gain. Bandwidth also is a function of closed loop gain. The non-inverting gain, i.e., noise gain, is calculated as EQU Gn=(Rin+Rfb)/Rin.
When connected as a variable feedback element the multiplying DAC output is connected to the amplifier inverting input. In this configuration the DAC output resistance Rout parallels Rin, increasing the noise gain Gn. The gain due to undesirable input error voltages and noise is now EQU Gn=((Rin//Rout)+Rfb)/(Rin//Rout).
DAC output resistance Rout is not constant, but varies as a function of digital control code. The magnitude of Rout variation may range from 0.75 times the DAC characteristic resistance Rdac to near infinity. Wide variations in Rout thus result in wide variations in noise gain Gn. For example, if Rin=Rfb=1 Mohm and Rdac=10 kohm, then Gn varies between 2 and 135.5. For every millivolt of input noise, input offset voltage or input offset voltage drift the output magnitude ranges between 2 mV and 135.5 mV. Additionally because the product of gain times bandwidth is constant, the variations in Gn cause corresponding changes in bandwidth.
What is desired is a method of adjusting operational amplifier gain to reduce undesirable changes in bandwidth, output offset voltage and output noise due to gain change resulting from changes in a digital control code input to a DAC used to adjust the gain of the operational amplifier.